This invention relates generally to the metal forming art, and more particularly to a method of and apparatus for the continuous casting of molten metal in a wheel-band type continuous machine.
The continuous casting of molten metal in the peripheral groove of a rotatable casting wheel is well known in the metal foundry art. Apparatus adapted to this type of casting usually includes a rotatable casting wheel having a casting groove formed in the periphery thereof which is closed along a portion of its length by a flexible endless band forming a casting mold therewith. A cooling medium is provided about the mold for developing a cooling gradient across the molten metal so as to accomplish solidification thereof.
While the casting of relatively pure metals such as copper and aluminum entails relatively uncomplicated cooling considerations, the casting of alloys, such as copper base alloys, aluminum base alloys and various ferrous alloys, entails more closely controlled and precise cooling considerations owing to the various metallurgical effects occasioned by variations in the cooling gradient thereacross.
It should be apparent, therefore, that when using the same casting machine to cast either different metals, or different alloys of the same metal, an adjustment has to be made to the casting machine in order to correct for variations in the required cooling gradients. This adjustment depends upon the type of alloy being cast, as well as the thermal conductivity thereof.
Because there are presently such a great number and variety of alloys desired to be cast, a need has arisen in the art for a casting machine that can be easily and readily adjusted to accommodate these metals. The primary adjustment of the casting machine is in the casting speed, which is easily adjusted. Another important adjustment, but one which is not so easily accomplished, is in the cooling efficiency which, by means of prior art systems, is controlled by varying the pressure and volume of flow of the cooling fluid. Since various alloys have different thermal conductivities, the cooling efficiency of the casting machine must be readily adjustable in order to cast different blends of metals on the same casting machine.
While the prior art technique of varying the cooling efficiency by controlling the pressure and volume of cooling fluid flow is acceptable when casting pure metals such as aluminum and copper, it does not achieve a sufficiently precise control necessary for varying the coolant gradient across various alloys where the coolant rate and distribution is more critical than the coolant rate and distribution for pure metals. Accordingly, it has been found desirable to vary the cooling efficiency by changing the length of the casting mold according to the cooling gradient desired. This can be conventionally accomplished by simple varying the degree of band wrap or angle through which the flexible band covers the casting groove. Increasing the amount of band wrap increases the length of the mold, while decreasing the amount of band wrap decreases the length of the mold.
Furthermore, when casting alloys at a high rate of speed it is apparent that the cooling efficiency becomes even more critical. As explained in U.S. Pat. No. 3,623,535, issued Nov. 30, 1971, to George E. Lenaeus et al., and assigned to the assignee of this invention, in the casting of molten metal in a rotatable casting wheel, the metal cools and solidifies in three distinct phases. The first phase begins when the metal is fed into the peripheral groove of the casting wheel and includes that portion of the casting process during which the metal is being cooled but is completely liquid within the casting wheel so as to be in complete contact with the surfaces of the casting mold. The second phase is that portion of the casting process during which the continued cooling of the metal causes an outer crust of solidified metal to form adjacent the surfaces of the casting mold, but during which the metal is still in substantially complete contact therewith. The third phase is that portion of the casting process beginning generally at or near that point in the solidification of the metal at which the continued cooling of the metal and thickening of the outer crust of solidilfied metal causes the metal to shrink away from the casting wheel and is that portion during which an air gap forms between the metal and the casting wheel.
The third solidification phase is the most troublesome inasmuch as the air gap between the metal and the casting surfaces greatly reduces the rate of heat transfer from the metal. In order to overcome this problem, it is disclosed in the aforementioned U.S. Pat. No. 3,623,535 to remove the flexible band from the casting wheel to expose the metal for direct cooling during some or all of the third solidification phase, thereby allowing the rotational speed and thus the casting rate of the casting wheel to be greatly increased since the effect of the air gap between the metal and the casting wheel is eliminated and the metal is exposed for direct cooling.
In veiw of the foregoing, it should be apparent that it is advantageous to provide a casting machine wherein the length of the casting mold is variable, not only to accommodate the casting of various metals and metal alloys, but also to permit the casting of alloys at a high rate of speed by means of the technique disclosed in the aforementioned U.S. Pat. No. 3,623,535, i.e., by shortening the angle of band wrap such that the peripheral groove in the casting wheel is covered only during the first and second phase of metal solidification. While the prior art has developed wheel-band casting machines wherein the casting band may be removed during shutdown of the machine and a shorter or longer band reinserted, with concomitant changes in the positions of the guide wheels, so as to change the length of the casting mold, there remains a need in the art for a casting machine wherein the length of the casting mold may be changed without replacing the casting band, and wherein this operation can be accomplished during the actual casting operation, as well as when the machine is shut down.
Additionally, when a continuous casting operation is first started, the casting rate (i.e., the rotational speed of the casting wheel) is maintained at a relatively low level concomitant with the other system parameters (e.g., temperature of the metal and the wheel, coolant pressure and volume, pouring rate, etc.). However, after the start-up period and when the casting machine is continuously operating at an equilibrium condition, the casting rate is then increased. Consequently, the cooling rate must also be increased to maintain the cooling gradient at the increased casting rate. This is, of course, conventionally accomplished by increasing the pressure and volume of coolant flow. Although it would be advantageous to increase the cooling rate by removing some of the band wrap to thereby subject the cast bar to direct spray cooling, this cannot be accomplished with prior art machines which have no means for changing the length of the mold (i.e., the degree of arc that the band covers the peripheral groove) during operation of the casting machine.